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diff --git a/doc/src/sgml/html/app-pgrewind.html b/doc/src/sgml/html/app-pgrewind.html new file mode 100644 index 0000000..9697fac --- /dev/null +++ b/doc/src/sgml/html/app-pgrewind.html @@ -0,0 +1,208 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>pg_rewind</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="app-pgresetwal.html" title="pg_resetwal" /><link rel="next" href="pgtestfsync.html" title="pg_test_fsync" /></head><body id="docContent" class="container-fluid col-10"><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center"><span xmlns="http://www.w3.org/1999/xhtml" class="application">pg_rewind</span></th></tr><tr><td width="10%" align="left"><a accesskey="p" href="app-pgresetwal.html" title="pg_resetwal">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="reference-server.html" title="PostgreSQL Server Applications">Up</a></td><th width="60%" align="center">PostgreSQL Server Applications</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="pgtestfsync.html" title="pg_test_fsync">Next</a></td></tr></table><hr></hr></div><div class="refentry" id="APP-PGREWIND"><div class="titlepage"></div><a id="id-1.9.5.9.1" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle"><span class="application">pg_rewind</span></span></h2><p>pg_rewind — synchronize a <span class="productname">PostgreSQL</span> data directory with another data directory that was forked from it</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><div class="cmdsynopsis"><p id="id-1.9.5.9.4.1"><code class="command">pg_rewind</code> [<em class="replaceable"><code>option</code></em>...] { <code class="option">-D </code> | <code class="option">--target-pgdata</code> }<em class="replaceable"><code> directory</code></em> { <code class="option">--source-pgdata=<em class="replaceable"><code>directory</code></em></code> | <code class="option">--source-server=<em class="replaceable"><code>connstr</code></em></code> } </p></div></div><div class="refsect1" id="id-1.9.5.9.5"><h2>Description</h2><p> + <span class="application">pg_rewind</span> is a tool for synchronizing a PostgreSQL cluster + with another copy of the same cluster, after the clusters' timelines have + diverged. A typical scenario is to bring an old master server back online + after failover as a standby that follows the new master. + </p><p> + After a successful rewind, the state of the target data directory is + analogous to a base backup of the source data directory. Unlike taking + a new base backup or using a tool like <span class="application">rsync</span>, + <span class="application">pg_rewind</span> does not require comparing or copying + unchanged relation blocks in the cluster. Only changed blocks from existing + relation files are copied; all other files, including new relation files, + configuration files, and WAL segments, are copied in full. As such the + rewind operation is significantly faster than other approaches when the + database is large and only a small fraction of blocks differ between the + clusters. + </p><p> + <span class="application">pg_rewind</span> examines the timeline histories of the source + and target clusters to determine the point where they diverged, and + expects to find WAL in the target cluster's <code class="filename">pg_wal</code> directory + reaching all the way back to the point of divergence. The point of divergence + can be found either on the target timeline, the source timeline, or their common + ancestor. In the typical failover scenario where the target cluster was + shut down soon after the divergence, this is not a problem, but if the + target cluster ran for a long time after the divergence, its old WAL + files might no longer be present. In this case, you can manually copy them + from the WAL archive to the <code class="filename">pg_wal</code> directory, or run + <span class="application">pg_rewind</span> with the <code class="literal">-c</code> option to + automatically retrieve them from the WAL archive. The use of + <span class="application">pg_rewind</span> is not limited to failover, e.g., a standby + server can be promoted, run some write transactions, and then rewound + to become a standby again. + </p><p> + After running <span class="application">pg_rewind</span>, WAL replay needs to + complete for the data directory to be in a consistent state. When the + target server is started again it will enter archive recovery and replay + all WAL generated in the source server from the last checkpoint before + the point of divergence. If some of the WAL was no longer available in the + source server when <span class="application">pg_rewind</span> was run, and + therefore could not be copied by the <span class="application">pg_rewind</span> + session, it must be made available when the target server is started. + This can be done by creating a <code class="filename">recovery.signal</code> file + in the target data directory and by configuring a suitable + <a class="xref" href="runtime-config-wal.html#GUC-RESTORE-COMMAND">restore_command</a> in + <code class="filename">postgresql.conf</code>. + </p><p> + <span class="application">pg_rewind</span> requires that the target server either has + the <a class="xref" href="runtime-config-wal.html#GUC-WAL-LOG-HINTS">wal_log_hints</a> option enabled + in <code class="filename">postgresql.conf</code> or data checksums enabled when + the cluster was initialized with <span class="application">initdb</span>. Neither of these + are currently on by default. <a class="xref" href="runtime-config-wal.html#GUC-FULL-PAGE-WRITES">full_page_writes</a> + must also be set to <code class="literal">on</code>, but is enabled by default. + </p><div class="warning"><h3 class="title">Warning</h3><p> + If <span class="application">pg_rewind</span> fails while processing, then + the data folder of the target is likely not in a state that can be + recovered. In such a case, taking a new fresh backup is recommended. + </p><p> + As <span class="application">pg_rewind</span> copies configuration files + entirely from the source, it may be required to correct the configuration + used for recovery before restarting the target server, especially if + the target is reintroduced as a standby of the source. If you restart + the server after the rewind operation has finished but without configuring + recovery, the target may again diverge from the primary. + </p><p> + <span class="application">pg_rewind</span> will fail immediately if it finds + files it cannot write directly to. This can happen for example when + the source and the target server use the same file mapping for read-only + SSL keys and certificates. If such files are present on the target server + it is recommended to remove them before running + <span class="application">pg_rewind</span>. After doing the rewind, some of + those files may have been copied from the source, in which case it may + be necessary to remove the data copied and restore back the set of links + used before the rewind. + </p></div></div><div class="refsect1" id="id-1.9.5.9.6"><h2>Options</h2><p> + <span class="application">pg_rewind</span> accepts the following command-line + arguments: + + </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="option">-D <em class="replaceable"><code>directory</code></em></code><br /></span><span class="term"><code class="option">--target-pgdata=<em class="replaceable"><code>directory</code></em></code></span></dt><dd><p> + This option specifies the target data directory that is synchronized + with the source. The target server must be shut down cleanly before + running <span class="application">pg_rewind</span> + </p></dd><dt><span class="term"><code class="option">--source-pgdata=<em class="replaceable"><code>directory</code></em></code></span></dt><dd><p> + Specifies the file system path to the data directory of the source + server to synchronize the target with. This option requires the + source server to be cleanly shut down. + </p></dd><dt><span class="term"><code class="option">--source-server=<em class="replaceable"><code>connstr</code></em></code></span></dt><dd><p> + Specifies a libpq connection string to connect to the source + <span class="productname">PostgreSQL</span> server to synchronize the target + with. The connection must be a normal (non-replication) connection + with a role having sufficient permissions to execute the functions + used by <span class="application">pg_rewind</span> on the source server + (see Notes section for details) or a superuser role. This option + requires the source server to be running and not in recovery mode. + </p></dd><dt><span class="term"><code class="option">-R</code><br /></span><span class="term"><code class="option">--write-recovery-conf</code></span></dt><dd><p> + Create <code class="filename">standby.signal</code> and append connection + settings to <code class="filename">postgresql.auto.conf</code> in the output + directory. <code class="literal">--source-server</code> is mandatory with + this option. + </p></dd><dt><span class="term"><code class="option">-n</code><br /></span><span class="term"><code class="option">--dry-run</code></span></dt><dd><p> + Do everything except actually modifying the target directory. + </p></dd><dt><span class="term"><code class="option">-N</code><br /></span><span class="term"><code class="option">--no-sync</code></span></dt><dd><p> + By default, <code class="command">pg_rewind</code> will wait for all files + to be written safely to disk. This option causes + <code class="command">pg_rewind</code> to return without waiting, which is + faster, but means that a subsequent operating system crash can leave + the synchronized data directory corrupt. Generally, this option is + useful for testing but should not be used on a production + installation. + </p></dd><dt><span class="term"><code class="option">-P</code><br /></span><span class="term"><code class="option">--progress</code></span></dt><dd><p> + Enables progress reporting. Turning this on will deliver an approximate + progress report while copying data from the source cluster. + </p></dd><dt><span class="term"><code class="option">-c</code><br /></span><span class="term"><code class="option">--restore-target-wal</code></span></dt><dd><p> + Use <code class="varname">restore_command</code> defined in the target cluster + configuration to retrieve WAL files from the WAL archive if these + files are no longer available in the <code class="filename">pg_wal</code> + directory. + </p></dd><dt><span class="term"><code class="option">--debug</code></span></dt><dd><p> + Print verbose debugging output that is mostly useful for developers + debugging <span class="application">pg_rewind</span>. + </p></dd><dt><span class="term"><code class="option">--no-ensure-shutdown</code></span></dt><dd><p> + <span class="application">pg_rewind</span> requires that the target server + is cleanly shut down before rewinding. By default, if the target server + is not shut down cleanly, <span class="application">pg_rewind</span> starts + the target server in single-user mode to complete crash recovery first, + and stops it. + By passing this option, <span class="application">pg_rewind</span> skips + this and errors out immediately if the server is not cleanly shut + down. Users are expected to handle the situation themselves in that + case. + </p></dd><dt><span class="term"><code class="option">-V</code><br /></span><span class="term"><code class="option">--version</code></span></dt><dd><p>Display version information, then exit.</p></dd><dt><span class="term"><code class="option">-?</code><br /></span><span class="term"><code class="option">--help</code></span></dt><dd><p>Show help, then exit.</p></dd></dl></div><p> + </p></div><div class="refsect1" id="id-1.9.5.9.7"><h2>Environment</h2><p> + When <code class="option">--source-server</code> option is used, + <span class="application">pg_rewind</span> also uses the environment variables + supported by <span class="application">libpq</span> (see <a class="xref" href="libpq-envars.html" title="33.14. Environment Variables">Section 33.14</a>). + </p><p> + The environment variable <code class="envar">PG_COLOR</code> specifies whether to use + color in diagnostic messages. Possible values are + <code class="literal">always</code>, <code class="literal">auto</code> and + <code class="literal">never</code>. + </p></div><div class="refsect1" id="id-1.9.5.9.8"><h2>Notes</h2><p> + When executing <span class="application">pg_rewind</span> using an online + cluster as source, a role having sufficient permissions to execute the + functions used by <span class="application">pg_rewind</span> on the source + cluster can be used instead of a superuser. Here is how to create such + a role, named <code class="literal">rewind_user</code> here: +</p><pre class="programlisting"> +CREATE USER rewind_user LOGIN; +GRANT EXECUTE ON function pg_catalog.pg_ls_dir(text, boolean, boolean) TO rewind_user; +GRANT EXECUTE ON function pg_catalog.pg_stat_file(text, boolean) TO rewind_user; +GRANT EXECUTE ON function pg_catalog.pg_read_binary_file(text) TO rewind_user; +GRANT EXECUTE ON function pg_catalog.pg_read_binary_file(text, bigint, bigint, boolean) TO rewind_user; +</pre><p> + </p><p> + When executing <span class="application">pg_rewind</span> using an online + cluster as source which has been recently promoted, it is necessary + to execute a <code class="command">CHECKPOINT</code> after promotion such that its + control file reflects up-to-date timeline information, which is used by + <span class="application">pg_rewind</span> to check if the target cluster + can be rewound using the designated source cluster. + </p><div class="refsect2" id="id-1.9.5.9.8.4"><h3>How It Works</h3><p> + The basic idea is to copy all file system-level changes from the source + cluster to the target cluster: + </p><div class="procedure"><ol class="procedure" type="1"><li class="step"><p> + Scan the WAL log of the target cluster, starting from the last + checkpoint before the point where the source cluster's timeline + history forked off from the target cluster. For each WAL record, + record each data block that was touched. This yields a list of all + the data blocks that were changed in the target cluster, after the + source cluster forked off. If some of the WAL files are no longer + available, try re-running <span class="application">pg_rewind</span> with + the <code class="option">-c</code> option to search for the missing files in + the WAL archive. + </p></li><li class="step"><p> + Copy all those changed blocks from the source cluster to + the target cluster, either using direct file system access + (<code class="option">--source-pgdata</code>) or SQL (<code class="option">--source-server</code>). + Relation files are now in a state equivalent to the moment of the last + completed checkpoint prior to the point at which the WAL timelines of the + source and target diverged plus the current state on the source of any + blocks changed on the target after that divergence. + </p></li><li class="step"><p> + Copy all other files, including new relation files, WAL segments, + <code class="filename">pg_xact</code>, and configuration files from the source + cluster to the target cluster. Similarly to base backups, the contents + of the directories <code class="filename">pg_dynshmem/</code>, + <code class="filename">pg_notify/</code>, <code class="filename">pg_replslot/</code>, + <code class="filename">pg_serial/</code>, <code class="filename">pg_snapshots/</code>, + <code class="filename">pg_stat_tmp/</code>, and <code class="filename">pg_subtrans/</code> + are omitted from the data copied from the source cluster. The files + <code class="filename">backup_label</code>, + <code class="filename">tablespace_map</code>, + <code class="filename">pg_internal.init</code>, + <code class="filename">postmaster.opts</code>, and + <code class="filename">postmaster.pid</code>, as well as any file or directory + beginning with <code class="filename">pgsql_tmp</code>, are omitted. + </p></li><li class="step"><p> + Create a <code class="filename">backup_label</code> file to begin WAL replay at + the checkpoint created at failover and configure the + <code class="filename">pg_control</code> file with a minimum consistency LSN + defined as the result of <code class="literal">pg_current_wal_insert_lsn()</code> + when rewinding from a live source or the last checkpoint LSN when + rewinding from a stopped source. + </p></li><li class="step"><p> + When starting the target, <span class="productname">PostgreSQL</span> replays + all the required WAL, resulting in a data directory in a consistent + state. + </p></li></ol></div></div></div></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="app-pgresetwal.html" title="pg_resetwal">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="reference-server.html" title="PostgreSQL Server Applications">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="pgtestfsync.html" title="pg_test_fsync">Next</a></td></tr><tr><td width="40%" align="left" valign="top"><span xmlns="http://www.w3.org/1999/xhtml" class="application">pg_resetwal</span> </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> <span xmlns="http://www.w3.org/1999/xhtml" class="application">pg_test_fsync</span></td></tr></table></div></body></html>
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